This invention relates to a color image formation method and a color image formation apparatus for reproducing colors according to the color material amounts of three or more colors including a color copier, a color printer, etc., and in particular to a color image formation method and a color image formation apparatus for determining an appropriate color material amount at the color reproduction time from an input color signal.
A color image formation apparatus places color material for forming an image on a recording medium on a recording medium by a method dependent on a unit, thereby forming an image. The color reproduction range is subject to limitations of the physical characteristics of the recording medium, color material, etc., the color material placement method, and the like. Generally, for full color printing, the three primary colors of cyan, magenta, and yellow or the three primary colors plus India ink are used.
On the other hand, a full color image may be independent of the limitations and actually contain a record-impossible color area outside the color reproduction range. For example, if an attempt is made to form a hard copy image based on the image displayed on a CRT unit, an area in which the color image cannot be reproduced may occur because the CRT unit differs from the image formation unit for forming the hard copy image in color reproduction range.
Thus, to reproduce an image containing color outside the color reproduction range of one image formation unit, an unreproducible color area needs to be replaced with a reproducible color area. Such operation is called color gamut compression. General techniques known as color gamut compression methods include a technique of replacing with color having the minimum color difference (coloriometric mapping), a technique of color gamut compression by dropping the saturation of the whole image (perceptual mapping), etc. For example, color outside the reproduction range is replaced with color having the minimum color difference in a unit (system) described in the Examined Japanese Patent Application Publication No. Hei 5-48657.
As another color gamut compression technique, a technique, for example, described in the Unexamined Japanese Patent Application Publication No. Hei 6-189121 is known to reproduce an image in only the three colors of cyan, magenta, and yellow. The technique assumes that an image displayed on a CRT unit is reproduced on a thermal sublimation printer for reproducing an image in the three colors of cyan, magenta, and yellow, and determines color gamut compression by using a difference evaluation function in response to an input signal for unreproducible color.
With recent color conversion technique development, such color gamut compression techniques have enabled color gamut compression with very high flexibility without giving side effects to colors in the color reproduction range.
To reproduce colors outside the color reproduction range by the conventional color gamut compression, attention is focused only on the shape of the color reproduction range and how colors outside the color reproduction range are compressed in the color reproduction range is considered. However, when actual color reproduction is executed, the lightness of a color mixture remarkably changes or if hue changes only a little, saturation remarkably changes. The reason why the problem arises is that the characteristics of color material used for actual image formation are not considered for compression into the color reproduction range.
For example, generally yellow color material can be reproduced in high lightness, but cyan and magenta color materials lower in lightness if the concentration is high or the area percentage is large. If colors are mixed, any color materials tend to lower in lightness. If the color reproduction range is compressed without considering such characteristics, the lightness remarkably changes or saturation remarkably lowers depending on hue.
A desirable compression method and evaluation function vary depending on whether reproduced color is light or dark as compared with input color or whether or not saturation is low although they equal in lightness. For example, to reproduce an image using only one yellow color material by controlling the area percentage on different printers, it in often desirable to reproduce the image in color that can be reproduced in the range in which colors are not mixed even if the color difference increases rather than to reproduce the image in color mixture having the minimum color difference.
Although there are the characteristics of the shape of the color reproduction range caused by color material actually used for recording, in the conventional color gamut compression techniques, a unified technique has been used in every hue direction as in the coloriometric mapping.
On the other hand, for example, green in reproduced by color mixture, thus it is desirable to reproduce color with a small color difference or lightness difference rather than to maintain the distribution of color materials making up the color mixture. For blue-family colors, particularly color mixture of cyan and magenta having low lightness is used, thus it is desired that high-lightness color outside the color reproduction range is reproduced with a lightness difference as small as possible. However, in the conventional color gamut compression techniques, attention is focused on reproduced color and input color and such distribution adjustment is not considered.